Some crankshaft is manufactured by being subjected to induction hardening and tempering followed by grinding. Induction hardening induces residual stress in steel. The residual stress causes cracks such as quenching cracks and grinding cracks. Tempering reduces residual stress and restrains the occurrence of cracks.
If a tempering process can be omitted, the manufacturing cost goes down. However, cracks attributable to the residual stress induced at the time of induction hardening are liable to occur. For this reason, there has been a demand for a steel for induction hardening in which cracks are less liable to occur even if a tempering process is omitted in the manufacturing process of a crankshaft.
JP61-186419A, JP2000-26933A, JP2005-256134A, and JP2007-113063A disclose steels which are used for manufacturing a hot forged product and a crankshaft, and in which cracks attributable to induction hardening are less liable to occur.
In the method for manufacturing a driveshaft disclosed in JP61-186419A, the occurrence of quenching cracks attributable to induction hardening is restrained by reducing the C content in a steel product. In JP61-186419A, the steel product further contains B to compensate the decrease in hardenability caused by the reduction in the C content.
In the steel for hot forging disclosed in JP2000-26933A, at least 0.04 wt % of Ti is contained to change the mode of sulfide, whereby the machinability after forging is improved, and the occurrence of grinding cracks at the time of grinding is restrained.
The steel material for crankshaft disclosed in JP2005-256134A contains at least 0.4 mass % of Si. A large amount of Si contained reduces the shrinkage at the time when carbides are produced by heat generated at the time of grinding, and restrains the occurrence of grinding cracks.
The hot-forged part disclosed in JP2007-113063A has a chemical composition such that a formula of fn2=521−353C−22Si−25Mn−8Cu−17Ni−18Cr−26Mo is at least 300. By making the formula of fn2 at least 300, the occurrence of quenching cracks is restrained.